1. Field of the Invention
The present invention relates to an oscillating circuit. More particularly, the invention relates to an oscillating circuit which improves the stability characteristics of the oscillating output signal. Applications of such an oscillator includes incorporation of the oscillator in radio wave key systems used for the remote control of automobile equipment.
2. Description of the Related Art
Recently, the use of radio systems utilizing radio waves for short distance transmission of information has increased dramatically. One example of this in the field of automobile related technology is the so-called "radio wave key system". Such systems typically operate by emitting a radio wave in the ultra high frequency (UHF) band from a transmitter incorporated in the ignition key. An automobile installed receiver then receives the radio wave and enables the control of such operations as, for example, door locking, engine starting, etc., at a location remote from the automobile.
The transmitters of these types of radio wave key systems typically incorporate an oscillator 52 that uses a crystal resonator 51 as shown in FIG. 3. The oscillator 52 oscillates the radio wave of several tens of MHz. A frequency multiplier 53 multiplies the oscillating frequency of the radio wave to obtain a desired high frequency (several hundreds of MHz). The high frequency thus obtained is amplified by an amplifier 54 in order to radiate it from an antenna 55.
Since the transmitter used in a radio wave key system is designed to be incorporated in the ignition key, it is desirable to keep the size of the transmitter as small as possible. More particularly, the number of the transmitter's component parts should be minimized and the structure of circuits should be kept simple in order to minimize the costs associated with the transmitter's manufacture. One response to the need for such a transmitter has in the past been the production of oscillators that utilize a surface acoustic wave resonator (hereinafter abbreviates as SAW resonator). SAW resonators have the advantage of small size and low cost over conventional electromagnetic structure.
The one port SAW resonator shown in FIG. 5 comprises a comb-shaped electrode 20 formed by a thin metallic film on a substrate made of a piezoelectric material, and a pair of grating reflectors 21 and 22 positioned on both sides of the electrode. An external terminal 23 is connected to the comb-shaped electrode 20. The electric signals input to the comb-shaped electrode 20 through the external terminal 23 are excited by the comb-shaped electrode 20, and transformed into elastic surface waves, and propagated along the surface of the substrate. Accordingly, the elastic surface waves are reflected by the grating reflectors 21 and 22 to generate the standing waves from the elastic surface waves. The standing waves are again converted by the comb-shaped electrode 20 into electric signals. In this way, it is possible to acquire sharp resonant characteristics for the resonator.
In oscillators utilizing SAW type resonators, a simple one transistor circuit may be used to oscillate a radio wave at a frequency of more than several hundreds of MHz. One advantage of using this type of one transistor circuit, is that the frequency multiplier 53, like that shown in FIG. 3, can be eliminated. As a result, it is possible to design a low cost extremely small transmitter by use of the oscillator which incorporates a SAW type resonator.
FIG. 4 is a view showing a conventional oscillator which uses a SAW resonator. The conventional oscillator 61 comprises an resonator SAW, a high frequency NPN transistor Tr, resistors R1 to R3, a variable capacitor (trimmer) C1, a fixed capacitor C2, and an antenna ANT.
The transistor Tr is grounded through the emitter resistor R3, and is biased at its base by means of resistors R1 and R2. The antenna ANT is connected to the collector side of the transistor Tr. When the collector current of the transistor Tr flows through the antenna ANT, radio waves are radiated from the antenna. This antenna ANT is often formed on a printed circuit board as a printed pattern. The capacitor C1 feeds back the emitter potential of the transistor Tr to the base. This design allows fine adjustments to be made to the oscillating frequency by varying the capacitance the capacitor C1.
The capacitor C2 feeds back the collector potential of the transistor Tr to the emitter. The inductance of the antenna ANT serves as alternating current load, to fluctuate the collector potential of the transistor Tr, assuming that the transistor Tr forms a common emitter grounded amplifier. This fluctuation is fed back to the emitter through the capacitor C2. Consequently, in order to achieve a stable feedback operation, it is necessary to acquire optimal antenna inductance as the AC load.
Now, the oscillator 61 tends to generate harmonics (secondary harmonics) at, two times that of the fundamental wave's desired oscillating frequency. The high level of the secondary harmonic may in fact create a problem, especially given regulations of some governmental agencies. For example, Japanese Radio Law requires that when a particular transmitter generates a signal at a frequency in excess of 320 MHz, the field intensity generated thereby should undergo a rapid decrease.
For example, if the fundamental wave is set at 160 MHz or more, the secondary harmonics (320 MHz) would not be within the regulated range of the field intensity in some cases. In particular, should a fundamental wave at approximately 300 MHz be transmitted from a radio wave key system, the secondary harmonic (at approximately 600 MHz) would most likely exceed the regulated field intensity.
Also, as described above, it is necessary to acquire optimal antenna inductance for the conventional oscillator 61. Consequently, the design of the oscillator 61 should allow the oscillating frequency to be fine tuned.
Antennas of the type described above, however, are usually arranged on a printed circuit board as a printed pattern. Consequently, it is impossible to modify the inductance once the printed circuit board is completed. One solution to this is to use a plurality of printed circuit boards each containing an antenna ANT having a different inductance. The circuit boards could then be prepared before assembling the oscillator. This, unfortunately, takes a great deal of time and work. Moreover, for those antennas which are arranged in patterns on printed circuit boards the actual inductance value of the antenna ANT is restricted by the configuration and size of the printed circuit board to be used. As a result, it is oftentimes difficult to design an antenna that can serve as an AC load and that provides for the fine tuning adjustment of the oscillator's oscillating frequency.